Strabismus is a misalignment of the visual axis, which can lead to severe deficiencies such as loss of central vision from one eye, known as amblyopia. Strabismus is relatively common in the general population with estimates of 5-6 percent. The etiology of strabismus is multifactorial. Current therapies for restoration of visual alignment include muscle weakening by surgical recession or pharmacological denervation with botulinum toxin and muscle tightening by surgical resection. In the proposed research project, the trophic regulation between eye muscles an innervating oculomotor neurons will be explored with the long-term goal to supplement surgical treatment of strabismus with a pharmacological treatment targeted at trophic interactions. Injections of trophic factors or trophic antagonists into selected eye muscles may restore balanced eye movements by mimicking intrinsic trophic mechanisms. The proposed studies will test in an animal model how trophic manipulations of oculomotor neurons and eye muscles can adjust the strength of these muscles, increase the survival of oculomotor neurons during development, increase numbers of collateral axonal branches of oculomotor neurons, and maintain axon collaterals and endplates. Studies will determine which trophic factors are produced in the eye muscles, which functions they have on muscle mass, muscle strength, nerve sprouting, and maintenance of axons or endplates. Additional studies will determine whether the muscle-derived factors are transported retrogradely to the oculomotor neurons and support the survival of these neurons. The time course of trophic interactions between eye muscles and their nerves will be explored with the goal to understand and manipulate the trophic responses which are induced by denervation with botulinum toxin or in chronically paralyzed muscle such as the avian genetic mutant, crooked neck dwarf (cn/cn). These studies will focus on four trophic factors, brain-derived neurotrophic factor (BDNF), glial cell-line-derived neurotrophic factor (GDNF), and the insulin-like growth factors (IGF I, II), and, added in the resubmission, cardiotrophin-1 (CT-1). Additional trophic factors will be screened for their potential to modify the strength of eye muscles. A combined pharmacological, molecular, physiological and morphological approach including the ultrastructural level will provide a meaningful assessment of the prospects for a trophic, pharmacological treatment of strabismus and other eye muscle disorders as a supplement to current resection and denervation procedures.